Method for the formation of diffusion superficial alloys, in particular chromium alloys



2,874,070 ION SUPERFICIAL UM ALLOlgS Feb. 17, 1959 P. GALMICHE OR THEFORMATION'OF DIFFUS LOYS, IN PARTICULAR CHROMI Sheets-Sheet 1 METHOD F'AL Filed Oct; 29. 1954 INVENTUR PHILIPPE GALNICHE BY 6 a ATTORNEYnmurwrwmq L av P. GALMICHE 2,874,070 N SUPERFICIAL ALL RMATION OFDIFFUSIO PARTICULAR CHROMIUM OYS v 5 Sheets-Sheet 2 Feb. 17, 1959 FORTHE F0 LLOYS, IN

METHOD A Fil ed Oct. 29, 1954 /NVENTDR PHILIPPE BALMICH E M 'ATTDPINEY17, 1959 F GALMICHE 2,874,070

METHOD FOR THE FORMATION OF DIFFUSION SUPERFICIAL ALLOYS, IN PARTICULARCHROMIUM ALLOYS Filed 001;. 29, 1954 5 Sheets-Sheet 3 INVENTDR PHI IPPECALMICHE y/ AT TDHNEY I Feb.'17, 1959 Filed 001;. 29. 1954 P. GALMICHEMETHOD FOR THE FORMATION OF DIFFUSION SUPERF'ICIAL ALLOYS, IN PARTICULARCHROMIUM ALLOYS 5 Sheets-Sheet 4 f w i w 9 r l ET 1 W I I L l.l A (D Ygnu m nu 5 \J um um 1 H \\H v \\H 22 INVENTDR PHILIPPE GALMIEHE EzY2,874,070 IFFUSION SUPERFICIAL Feb. 17, 1959 I P. GALMICHE I METHOD FORTHE FORMATIONOF D ALLOYS, IN PA M ALLOYS RTICIJLAR- CHROMIU 5Sheets-Shae Filed Oct. 29, 1954 Fig];

INVENTUR PHILIPPE BALM/CHE ATTUHNFY United States Patent METHOD FOR THEFORMATION OF DIFFU- SION SUPERFICIAL ALLOYS, IN PARTICU- LAR CHROMIUMALLOYS Philippe Galmiche, Paris, France, assignor to Office NationaldEtudes et de Recherches Aeronautiques, 'Chatillon-sous-Baqneux, France,a society of France Application October 29, 1954, Serial No. 465,657Claims priority, application France May 16, 1951 so Claims. or. 117-107addition metals.

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, superficial hardness, when the pieces to be treated are of small size.

On the contrary, when these pieces have an important mass and/ or a highthermal conductivity, it may happen, in particular at the beginning ofthe treatment, that condensations of the halide or halides of theaddition metal or metals are formed on the pieces to be treated.

The object of the present invention is to provide methods and apparatusfor the formation of such alloys which are better adapted to meet therequirements of practice than those known up to the present time.

The invention relates to the case where the diifusion alloy is obtainedby subjecting the piece to be treated to the action of at least onehalide of an addition metal, obtained from a suitable cementingsubstance, and its essential feature consists in preventing condensationof said. halide on the piece in treatment and also, if

- necessary, on anyregeneration metal as may be in contact with saidpiece. a

Preferred embodiments of the present invention will be hereinafterdescribed with reference to the accompanying drawings, given merely byway of example, and in which: i

Fig. 1 is a diagrammatic vertical section of -a furnace for theformation of a superficial alloy according to my invention;

Figs. 2, 3, 4, 5 and 6 show other furnaces according to my invention;

Fig. 7 shows, on an enlarged scale and in vertical section, a reactioncase to be used in the furnace shown by Fig. 6;

Fig. 8 diagrammatically shows a continuous chromizing furnace forcarrying out my invention;

Fig. 9 shows, on an enlarged scale and in elevational view partly insection, a reaction case intended to be circulated through the furnaceof Fig. 8;

.Figs. 10 and 11 are diagrammatical views of two other furnaces forcarrying out my invention.

In what follows, it will be supposed, as a rule, that it is desired toform on ferrous pieces 1 superficial diffusion alloys includingchromium.

It is known that intermetallic diffusion alloys may be obtained by theaction on the pieces to be treated of halides of the addition metals andin particular of the fluorides of these metals, intermetallic ditfusiontaking place either in a purely gaseous phase, that is to say with thepieces wholly out of contact both with the cementing mixture whichsupplies said metals and with any metal which may be used forregenerating active vapors, or according to what is called thesemi-contact process, i. e. with the pieces in contact with aregeneration metal in the divided state, the vapors of halide or halidesof the addition metals being caused to pass through this mass ofregeneration metal.

Such methods make it possible to obtain perfectly bright surfaces, withan' even protective layer of good These condensations result, .at theend of the treatment, in the formation of dull zones on pieces 1, and,in the case of the semi-contact process, in the fact that theregeneration metal sticks to the surface of said pieces.

It was found that most of these detrimental effects take place duringthe period of heating up of the reaction chamber to the temperature ofoperation. 7

I will therefore first examine the application-of the present inventionto this period.

According to this invention, and in order to avoid the drawbacks abovementioned, I proceed in such manner that, at least from the time whenthe halide of the addition metal has, in the vicinity of the pieces tobe treated, a vapor tension capable of giving rise to detrimentalcondensations on the pieces in course of treatment, the surfacetemperature of said pieces is not more than 200 C. below the temperatureof the cementing mixture and is preferably higher than said temperature.

For the sake of simplicity, it will be hereinafter supposed that thetreatment is performed with a single halide.

In a general manner, I may, according to the present invention, eithercause the halide vapors to condense on a filter layer advantageouslyconstituted by a metal in a divided state and of high conductivity, sothat the heating of pieces 1 up to the desired temperature can takeplace without contact with halide vapors at a higher temperature, orsubject the pieces to be treated to a. preliminary heating, or againkeep the cementing mixture at a temperature lower than the surfacetemperature of said pieces In the first case, I may dispose, between thecementing mixture and the ,first pieces to be treated, a sufficientlayer of a metallic material in big lumps, on which condensation takesplace, before the halide can reach the insufficiently hot pieces to betreated. 7

- I might also, when the treatment is to be applied simultaneously bothto light pieces (for instance clockwork elements) and to heavier pieces,instead of providing a special filter layer, place the heavier pieces inregions of the chamber of treatment at a greater distance from thereserves of cementing mixture than the regions where the lighter piecesare located.

In this case, preferably, I make use of the I gaseous phase process forthe lighter pieces and of the semicontact process for the bigger pieceswhich suffer less risk of being deformed in contact with the lumps ofregenerating metal.

The preheating solution is advantageously employed, in case of a purelygaseous phase treatment,.by subjecting the pieces to be treated to anindependent preheating in a neutral or reducing atmosphere.

On the contrary, when the treatment is carried out with the semi-contactprocess, it will be advantageous to proceed to the preheating, insuitable vessels and in a neutral or reducing atmosphere, of both thepieces to be treated and the metallic reserve of the addition metals. Asfor the third solution (suitable adjustment of the temperature of thecementing mixture), it may be applied, either by providing, between theheating wall and the cementing mixture, an intermediate screen of lowthermal conductivity, or by heating the'chamber of treatment insuch'manner that the pieces of greater mass are heated first, thecementing mixture being brought to high temperature only when all risksof condensation of the halide have disappeared, or again by limiting therate of heating of the cementing mixture by addition of water or ahalogen acid in aqueous solution, the amount thereof being higher thanthat necessary for the formation of the desired amount of halide of theaddition metal, the latent heat of vaporization of water limiting thevapor tension of the halide until the diffusion equilibrium has beenreached.

Fig. 1 shows, merely by way of example, an apparatus for applying someof the features above stated.

The cementing mixture is disposed at A at the lower part of a removablecase 2. The sweeping gases (hydrogen, ammonia, either cracked or not,for instance) flow in through a conduit 3 and out through a conduit 4,these conduits being fixed while case 2 is interchangeable.

The lighter pieces are treated in a purely gaseous phase in a perforatedcage 15, whereas the heavier pieces are embedded in a reserve'B ofchromium or ferro-chromium in the form of big lumps (semi-contactprocess) in which may be incorporated some neutral ammonium fluoride toproduce a sweeping gas stream, and possibly molybdenum.

Furthermore, the pieces of higher thermal capacity are located at agreater distance from cementing mixture A than the lighter pieces, alayer C of ferro-chromium in the divided state being possibly providedto promote, between said cementing mixture and the lighter pieces,condensation of the halide during the heating period.

On the other hand, the heating elements 16 act upon the side wall andthe upper wall of case 2 but not on the bottom thereof, which supportsthe cementing mixture A, which is favorable for the proper preliminaryheating of pieces 1. In order to obtain a preheating of the heavierpieces 1, I may also distribute the heating elements 16 in severalgroups which are separately fed wtih current, the elements whichcorrespond to the portion of the heating case where are located theheavier pieces being then fed first.

If it is merely desired to chromize, in a purely gaseous phase,relatively small pieces, for instance clockwork pieces, it may besufficient to preheat these pieces in a neutral atmosphere beforesubjecting them to the action of the chromium halide (preferablyfluoride) vapors.

For this purpose, it is advantageous to have recourse to the apparatusillustrated by Fig. 2, which diagram-. matically shows a verticaltubular furnace of a refractory alloy, the inner diameter of which maybe relatively small, for instance averaging 10 centimeters and insidewhich are provided three distinct zones of treatment, to wit, going fromtop to bottom, a cooling zone I of relatively great length, a preheatingzone II, heated, for instance electrically, to a temperature rangingfrom 1100 to 1200 C. by heating elements 16a, and a chromizing zone IIIheated to a temperature ranging from 1150 to 1300 by heating elements16b which also heat the bottom of the tubular furnace.

Sweeping may be performed by means of a mixture (heavier than air) ofargon and cracked (or noncracked) ammonia, this mixture flowing inthrough conduit 3a preferably at the junction between zones I and IIwhereas it flows out through the top of the furnace.

A reserve of chromium or ferro-chromium B is provided on the peripheryof zone III. Through a tube 17, there is introduced successivelyammonium fluoride to drive out the air at the beginning of thetreatment, then chromium fluoride in small amounts during the whole ofthe operation. In view of its high density, chromium fluoride willremain in zone 'III and will mix but little with the sweeping gases ofzones II and I.

The operation is very simple. Pieces 1 are preheated in zone II for someminutes, then lowered into the chromizing zone III where they stay for atime ranging from some minutes to one hour (position shown in solidlines).

They are then cooled in zone I for the necessary time (position shown indotted lines).

I will now describe, still by way of example, a furnace, of general Ushape, for chromizing, without risks of halide condensation, pieces 1treated according to the semicontact process and in semi-continuousmanner.

This furnace, diagrammatically illustrated by Fig. 3, essentiallyincludes three portions, to wit: a preheating zone constituted by avertical branch 18 of the furnace, into which the cases are introducedand where they are heated at a temperature of about 1000 C.; ahorizontal chromizing zone 19, heated at about 1050, through which thecases are passed; and a cooling zone, constituted by the other verticalbranch 20 of the furnace, serving to the exit of the cases.

The cases which areintroduced into the furnace are constituted bycylindrical boxes. The top 21 and the bottom 22 of these cases projectslightly from the periphery of the side walls and their diameter isnearly equal to the inner diameter of the vertical branches ofthefurnace in order to constitute therewith a relatively fluidtightjoint. The sidewalls of the cylinders are provided with holes andcontain the pieces to be treated, in contact with chromium B in thedivided state containing a small amount of neutral ammonium fluoride (aproduct which gives oil? a large volume of reducing gas and practicallydoes not react with chromium). The case includes, at its lower part, ahousing in which may be placed a container filled with the cementingmixture A. The total height of the case is greater than the height ofthe horizontal branch of the furnace so that, as soon as a case isintroduced into the preheating zone.18, the cementing mixture is alreadyin the chromizing zone 19.

The twovertical branches of the furnace may be closed by covers 23a and23b. 7

At one of the ends of the horizontal portion 19 of the furnace, there isadvantageously provided an orifice through which can be slidablydisplaced a bar 24 serving to circulate the cases.

The vertical branch 20 of the furnace, which serves to the cooling ofthe pieces, is separated from the heated part by a heat insulating joint25 and may be closed at its lower part by a gate of a refractorymaterial 26.

The .furnace is swept by a gas (ammonia for instance) entering both thechromizing zone, at the exit thereof, and the cooling zone. The inflowof this gas into these two furnace portions is controlled by valves 27and 28.

The operation of the furnace includes three distinct periods, to witpreheating, chromizing and cooling. The preheating period includes thefollowing operations:

Cover 23a is removed;

Case a, which was resting on case b, is lifted in branch Case b beingthus released and the end case n1 in pant 19 .having been, lifted intobranch 20, cases [2, c, etc., are pushed forward;

Case a is then lowered into the position previously occupied by case band a new case is introduced into branch 18 to occupy the positionpreviously occupied by a;

Cover 23a is placed back.

The case introduced into the preheating zone is in a reducing atmosphereand is brought to the desired temperature without coming into contactwith chromium fluoride, which is evolved exclusively in thechromizingzone.

The chromizing period takes place as follows:

The cases, introduced into the horizontal portion 19of the furnace areheated to a temperature ranging from 1050 to .1100. Gas sweepingproduces in the furnace a slight overpressurepreventing the, inflow ofoxidizing gases.

Finally, the cooling period includes the following operations:

when the cementing vnarilyv by the action ithecase of the semiacontactprocess) also before the lifted into the cooling Owing to this furnaceconstruction, it is possible to perform chromizing in a semi-continuousmanner.

For a charge of about 50 kgs., preheating requires about 45 minutes,chromizing 2 hours and a quarter of an hour, and cooling about 45minutes. It therefore suffices to provide a furnace in which one case isin heating position, three cases in chromizing position and one incooling position. v

In the preceding description, I have indicated various steps 'to betaken to avoid halide condensation on the pieces to be treated duringthe period for which the reaction chamber is heated up to the reactiontemperature.

If size in a purely gaseous phase, these steps lead to excellentresults.

On the contrary, when the treatment is of the semicontact type (thepieces being in contact with chromium intended toregenerate the halide),these steps are sometimes insuflicient and during the period of coolingof the reaction chamber some spots may appear upon pieces 1 and, what ismore important, the chromium used for regeneration may be soiled. Whenthis chromium is next used for another operation, the soiled spotsthereof will melt, spread and in turn spoil the new pieces treated.

This drawback may be avoided in one of the following manners.

A first solution consists in introducing only the strictly necessaryamounts of the addition halide or halides or of the elements which, incementing mixture A, give rise to these halides, so that, at the end ofthe treatment, the atmosphere contains just the amount imposed by thelaws governing diffusion. The addition halide or halides,

,or the elements which form them, may then be introthe course of thediffusion opduced either gradually in eration or wholly at the beginningthereof.

It should be noted that the reaction casesintroduced into the furnacemust be sufficiently fluidtight mixture has been introduced incontrolled amount at the beginning of the operation. In order to obtaina gradual and limited disengagement of the active vapors, in theparticular case of chromizing ferrous pieces, one may have recourse forinstance to the following steps: either to drop on chromium-located outof contact with the pieces anamount of ammonium fluoride carefullydetermined in accordance with the needs; or to constitute cement A by acontrolled amount of chromium fluoride obtained prelimion chromium of ametallic fluoride such as ammonium of liquid hydrofluoric acid, or againto subject chromium located out of contact with the pieces to the actionof fluorine or hydrofluoric acid in controlled amount, diluted ;ifnecessary in an inert or reducing gas, or again to subject a reserve ofchromium placed out of contact with the. pieces to the action of anauxiliary fluoride which, in a reducing atmosphere, disengageshydrofluoric acid orproduces an exchange reaction in the presence of-chromium, thus disengaging chromium fluoride and abandoning its metalin contact with chromium.

I. A second solution consists in cooling down the cementing mixture Abefore the pieces 1 to be treated and (in regeneration metal. B.

thetreatment is performed on pieces of medium taming, at the lower partthereof, the cementmg mixture instance at temperatures averaging 1000 Athird solution consists in introducing into the reaction chamber,advantageously in the vicinty of the cey nienting mixture, at the end ofthe treatment, a body of which is at a temperature lower than that ofthe at mosphere of the furnace, which body may either have a highthermal inertia or be kept at the desired temperature by circulation ofa cooling 'iluid. This body may be introduced either gradually or as awhole. The operation may be stopped and started again a plurality oftimes. Thus, the excess of halides contained in the reaction cases whenthe temperature is lowered at the end of the treatment will condense onthese cold points or inthe vicinity thereof. i Y According. to stillanother feature of my invention, when a portion of the reaction chamberis brought (for instance according to the above mentioned thirdsolution) to a temperature lower than the zone of the furnace where arelocated the pieces and the regeneration metal, this portion issurrounded, preferably completely, by a layer constituted exclusively bylumps of a conductor metal, preferably the addition metal, which aretherefore at a temperature intermediate between that of said cooledportion and that of the pieces and the regeneration metal, i. e. arecolder than said piecesand said regeneration metal.

This arrangement makes it halide condensations on the metal of thislayer. the regeneration metal which surrounds the pieces to be treatedis not soiled and can be utilized during a subsequent operation withoutrisks of direct contacts between the said pieces and the halide in thesolid or liquid state. It should be noted that this last mentionedarrangement makes it possible to obtain, at the end of anoperation, ametal (that contained in the second chamber) on which the halide hasconcentrated, this metal constituting, in the course of the nextoperation, a reserve of halides of the addition metal (for instance ofchromium fluoride) having a large area of gaseous disengagement owing towhich the atmosphere of the furnace is quickly homogenized. I

I will now describe, by way of example, some-furnaces for carrying outmy chromizing method as above set forth. c

Fig. 4 shows a furnace the lower portion I of which is kept at atemperature lower than that of the, other portions 11. These lastmentioned portions may be for C., the temperature of the lower portiondecreasing, near the bottom, from 1000 C. toabout 500 C. V

Advantageously, I provide a reducing gas sweeping, for instance by meansof hydrogen, the sweeping gas being fed to the furnace through a conduit3 located in the top portion of the furnace.

In order to carry out the diffusion treatment in this furnace, I makeuse of a fiuidtight reaction case 2 conpossible to localize the A (forinstance a mixture giving off chromium fluoride),

- the pieces 1 to be treated and the regeneration chromium ,60 fluoridein the presence of water or B being located above thiscementing mixture,

The treatment takes place as follows: At the beginning, case 2 isbrought into the position illustrated by the drawing and pieces 1, sameas chromium B, undergo a preheating with respect to the cement A locatedin the portion of thefurnace at lower temperature. Case 2 is then liftedto level h so that the whole is ata substantially homogeneoustemperature, cement A, which is nearer to zone I of the furnace, beinghowever at a temperature slightly lower than thatof pieces 1. This phasecorresponds to the chromizing treatment proper. Once this treatment isfinished, a precooling action (in order to prevent'halideJcondensationson pieces 1 andchro- Thus,

tion chromium B (this corresponding to the second solution abovementioned). Finally, general cooling is obtained by removing thereaction case 2 from the furnace (position shown in dot-and-dash lines).

Fig. 5 shows a vertical furnace including a central apertured shaft 19for the introduction, at any time, of the reactive elements and, inparticular, of a cementing mixture A containing the chromium halide,this shaft further serving, at the end of the operation, to introduce acold tube (third solution above mentioned). The pieces 1 to be treatedare disposed in baskets 20, in contact with lumps of regenerationchromium B.

Advantageously, shaft 19 is surrounded by an apertured sleeve 21 and theannular space between said shaft and said sleeve is filled with lumps ofchromium (or ferro-chromium) P which cause any possible condensation totake place thereon. Thus, no halide condensation takes place at the endof the Operation on pieces 1 or the regeneration chromium B in contactwith said pieces.

This furnace is then advantageously completed by heat insulation fins 22rigid with the cover 23 of the furnace and by a plug 24 for stoppingshaft 19.

Figs. 6 and 7 show another embodiment of a vertical furnace in which areintroduced ,fluidtight autonomous cases 25 containing each its ownreserve of cementing mixture, said cases being stacked one above theother, and a hollow tube 19a extending therethrough for the introductionof the element intended to localize halide condensations (a cooled tubefor instance).

Every case 25 contains, in addition to pieces 1 and regenerationchromium B which surrounds them, one or several apertured vessels 26containing a reserve of cementing mixture A surrounded by pieces ofchromium of ferro-chromium F playing the same part as the pieces ofchromium located, in the case of Fig. 5, between shaft 19 and sleeve 21.i

Fig. 8 diagrammatically shows a continuous chromizing furnace includinga main element 27 in which the temperature is substantially homogeneous,said element being located between two exchangers 28 and 2? servingrespectively to preheating (at the beginning of the operation) andprecooling (at the end).

Through these various elements extends a passage 39 in which circulatefluidtight cases 31 containing the pieces to be treated, these casesentering on the side of the preheatingelement 28 and being removed onthe side of the cooling element 29. 1

Every fiuidtight case 31 contains, in addition to the pieces 1 to betreated and the regeneration chromium B which surrounds said pieces, areserve of cementing mixture A located in the rear portiorrof the caseand close to the walls thereof, said reserve of cementing mixture Abeing separated from pieces 1 and the regeneration chromium B by areserve of lumps of chromium F for causing condensation to take placethereon, this reserve extending preferably around the walls of case 31and being separated from the regeneration chromium B by ,aperturedpartitions 32.

With such a furnace, the cycle of. operations is as follows: Case 31 isfirst introduced into exchanger 28,

where it undergoes a first heating, which concerns the whole case. It isthen pushed further into passage 30 until it partly engages heatingelement 27, pieces 1 and the regeneration chromium B then undergoing apreheating. It is then fully engaged into element 27 for the chromizingprocess proper. It is then wholly removed from element 27 and brought.to excharlgerzi where precooling takes place, the products closest tothe wall of case 3i (cementing mixture A and chromium reserve F) beingcooled more quickly and .more intensively than pieces 1 and-theregeneration chromium B. Finally, case 31 is withdrawn'from passage 30to undergo the general cooljug phase.

Advantageously, the cooling liquid arrives at the level of theportion ofthe passage (where it passes through exchanger 29) which is in contactwith the portion of case 31 where is located cementing mixture A.

According to still another feature of my invention, particularlyillustrated by Figs. 10 and 11, I establish every reaction casing 2 insuch manner that it is fluidtight with the exception of a leakage zone fintended to make it possible to balance the internal and externalpressures, which leakagezone is advantageously located in the portion ofthe casing which is most remote from the cementation mixture.

I then provide, around the reaction casing or casings, a neutral orreducing atmosphere, for instance containing a high percentage ofhydrogen, at least at the end of the treatment.

Preferably, according to my invention, during the heating up period, Ipreheat the mass constituted by the body or bodies to be treated and theregeneration metal, for instance chromium, in such manner that, on theone hand, the coldest points of said mass are brought to a temperaturewhich is at least C. and advantageously at least C. above thetemperature of the portion of the furnace where is located the halogencontaining reserve, and, on the other hand, that the hottest point ofsaid mass is at a temperature not more than 600 C. (and advantageously500 C.) above the temperature of the portion of the furnace where islocated the cementation mixture. It should be noted that such anintensive preheating is suflicient to ensure a perfect finish of thearticles, even if no special step is taken to deacidify the cementationmixture. As a matter of fact, in this case there is no risk, in view ofthe temperature range at which preheating is performed, of substantialamounts of solid or liquid chromium fluoride being formed by the actionof hydrofluoric acid on the regeneration chromium.

Consequently, it is even possible in these conditions to make use of acementation mixture which does not contain chromium but only a reservecapable of forming a halide (preferably a fluoride) of another metal,such a reserve forming vapors of the halide of this metal whichwill'react with the regeneration chromium to form vapors of chromiumhalide which serve to perform the desired chromi zing'of the articles.

Such a cement, which may be called an indirect cement, may beconstituted, for instance, in one of the following ways:

A first method consists in mixing a little volatile halide (for instancenickel fluoride) and a little volatile oxide (for instance tungstic.oxide), 'this mixture gradually evolving a halide vapor (tungstenfluoride) which itself yields, by contact with the regeneration"chromium, chromium fluoride. I might associate in the same manneraluminum or chromium fluoride with tungstic anhydride or moiybdicanhydride.

Another method consists in making use of products capable of graduallygenerating halogen ions, for instance hypo-halogenites having thegeneral formula MOX (for instance CaO C1 which, when heated, graduallyyield metal oxides (MO) and evolve the halo- It is then advantageous, inorder to increase the area of exchange between the hot gases andthe massto be preheated, to distribute the articles P, the regeneration chromiumR and the cementation mixture C in several independent casings orboxesl, for instance in two boxes as shown by Fig. 10, each box thenconstituting an independent treatment unit.

It may be advantageous to give special shapes to these boxes (forinstance annular boxes or boxes provided with passages) so as toincrease the area of exchange.

Figs. 1 0 and 11 show, by way of example, two examfples offurnaces foroperating according to my invention.

' (about of the charge).

' not disturb the balance of the cated remain on the outside.

' containers 2 may be introduced by means of an elevator hydrofluoricacid) in which concentrated liquid hydrofluoric acid is poured onto fineparticles of chromium to -which has been added a small amount ofammonium fluoride (about one hundred grams of commercial concentratedhydrofluoric acid for three hundred grams of chromium in particles ofthe size of rice grains, with the addition of from two to ten grams ofammonium fluoride).

The temperature is slightly raised and the reaction begins at atemperature of 25-30 C., accelerating when the mixture comes to atemperature of 50-80 C. It goes on for some minutes after which thetemperature is raised to 200-600 C. to eliminate the excess ofhydrofluoric acid and to avoid a subsequent active hydrolysis of thechromium fluoride that is formed (case where it is necessarysubsequently to add a small amount of water to the cementation mixturein order to decarburize the articles).

I then add to the mixture thus obtained from 1 to 3% of tungstic acid inorder to form a homogenizing halide and from 1 to 3% of ammoniumfluoride in order to eliminate the oxidizing atmosphere in the furnace.A

small jet of gaseous ammonia into the mixture completes deacidificationthereof. It is then possible, if necessary,

to add a small amount of water or ammonium carbonate to decarburize thearticles superficially without any practical risk of hydrolysis of thereserve of chromium fluoride.

In every treatment container of five litres, there is placed about fiftygrams of the cementation mixture A thus formed, and this cementationmixture is covered with a layer 36 of trapping chromium in rather biglumps The articles 1 to be treated and the regeneration chromium B arethen introduced in the respective propor- -tions of 60% and 40%(approximately) of the total charge. Hydrogen is circulated around thecontainers (or ammonia which is cracked at the treatment temperature)which gas will hardly penetrate into containers 2 until the end of thecooling thereof and which thus does chromizing atmosphere. For thepreheating period, the containers are incompletely introduced into thefurnace, so that the portions of said containers where the cementationmixture A is 10- It should be noted that the portion of the containerswhich is introduced into the furnace will be heated more efliciently asit is surrounded by a gas (hydrogen) which is a good conductor of heat.

At the end of the operation, the boxes or containers are brought backinto the position where they were placed to undergo preheating. Theportion of every i containerwhich is provided with a leakage zone isstill in a hydrogen atmosphere which is the only gas to be able thecasing during cooling. I thus obtain a precooling of the cementationmixture and of the trapping chromium, such a precooling achievingcondensation of the vapors on elements (trapping chromium andcementation mixture) other than the articles to be to penetrate intotreated and the regeneration chromium which must notbe soiled.

In a general manner, while I have, in the above description, disclosedwhat I deem to be practical and efiicient embodiments of my invention,it should be well 10 understood that I do not wish to be limited theretoas there mightbe changes made in the arrangement, disposition and formof the parts without departing from the principle of the presentinvention as comprehended within the scope of the accompanying claims.

. This application is a continuation-in-part of my U. S. applicationsSer. No. 287,989, of May 15, 1952 and Ser. No. 351,680, of April 28,1953, both now abandoned.

What I claim is:

l. A method of forming a superficial alloy of chromium on a metal bodyhaving as a principal component a metal selected from the groupconsisting of iron, nickel and cobalt comprising heating a cementationmixture capable of forming chromium fluoride vapors on heating, heatingsaid metal body to reach and maintain a temperature at which chromiumfrom said vapors will diffuse into the surface of said metal body,bringing said vapors in contact with, while maintaining said cementationmixture out of contact with, said metal body surface, and maintainingthe relative temperatures of said vapors and said metal body surfaceduring the heating of said metal body to reach said diffusiontemperature so as to prevent condensation of chromium fluoride on saidmetal body.

2. A method of forming a superficial alloy of chromium on a plurality ofmetal bodies of relatively small and large sizes and having as aprincipal component a metal selected from the group consisting of iron,nickel and cobalt comprising positioning in a chamber a cementationmixture capable of forming chromium fluoride vapors on heating,positioning said larger metal bodies in said chamber spaced from saidcementation mixture, positioning said smaller metal bodies in saidchamber spaced from said cementation mixture and between said largermetal bodies and said cementation mixture, heating said chamber to formsaid chromium fluoride vapors and to reach and maintain a temperature atwhich chromium from said vapors will diffuse into the surfaces of saidmetal bodies,

bringing said vapors in contact with said larger metal bodies by passingover said smaller metal bodies whereby said vapors do not reach contactwith said larger metal bodies until the temperature of said larger metalbodies has reached the point where said vapors will not condensethereon.

3. A method as claimed in claim 2 additionally comprising thepositioning of a regeneration mass of chromium in contact with saidlarger metal bodies.

4. A method of forming a superficial alloy of chromi um on a metal bodyhaving as a principal component a metal selected from the groupconsisting of iron, nickel and cobalt comprising placing in a chamber acementation mixture capable of forming chromium fluoride vapors onheating, heating said chamber to the temperature at which chromium fromsaid vapors will diffuse into the surface of said metal body, preheatingsaid metal body to a temperature above that where said vapors wouldcondense thereon, and introducing said preheated metal body into saidchamber spaced from said cementation mixture whereby said vapors in saidchamber are diffused into said body.

5. A method as claimed in claim 4 additionally comprising thepositioning of a regeneration mass of chromium in contact with saidmetal body.

6. A method of forming a superficial alloy of chromium on a metal bodyhaving as a principal component a metal selected from the groupconsisting of iron, nickel and cobalt comprising positioning said metalbody in a chamber, positioning in said chamber spaced from said body acementation mixture capable of forming chromium fluoride vapors onheating, heating said chamber to form said chromium fluoride vapors andto reach and maintain a temperature at which chromium from said vaporswill diffuse into the surface of said metal body surface, controllingthe heating of said cementation mixture so as to maintain thetemperature of said fluoride 1 1 vapors formed below the point wherethere will be substantial condensation of said vapors on said metal bodyduring the'heating of said cha'mber to reach said diffusion temperature.

7. A method as claimed in .claim 6 additionally comprising maintainingsuch controlled relationship of the temperature of said fluoride vaporsand of said metal body during the period of cooling down of saidchamber.

8. A method as claimed in claim 6 wherein the composition of saidcementation mixture is so regulated as to insure that the amount offluoride vapors formed therefrom shall not substantially exceed theproportion required to complete the diffusion reaction.

9. A method as claimed in claim 6 additionally comprising, at the end ofsaid diffusion, cooling said cementation mixture at a rate in relationto the temperature of said metal body so as to maintain the temperaturerelationship between said vapors and said metal body such that nosubstantial condensation of said vapors on said metal body will occur.

10. A method as claimed in claim 6 additionally comprising, at the endof said diffusion, introducing a cooling body within said chamber nearerto said cementation mixture than said metal body, said cooling bodyhaving a substantially lower temperature than the atmosphere of saidchamber.

11. A method of forming a superficial alloy of chromium on a metal bodyhaving as a principal component a metal selected from the groupconsisting of iron, nickel and cobalt comprising positioning said metalbody in a chamber, positioning in said chamber spaced from said body acementation mixture capable of forming chromium halide vapors onheating, positioning a regeneration mass of chromium in contact withsaid metal body, heating said chamber to form said chromium halidevapors and to reach and maintain a temperature at which chromium fromsaid vapors will ditiuse into the surface of said metal body, bringingsaid vapors in contact with said metal body surface, and maintaining therelative temperatures of said vapors and the surfaces of said metal bodyand said regeneration mass during the heating of said metal body toreach said difiiusion temperature so as to prevent condensation ofchromium hal de .on said surfaces of said metal body and regenerationmass.

12. A method as claimed in claim 11 wherein the temperature of saidsurfaces are maintained above 200 C. below the temperature of saidcementation mixture.

13. A method as claimed in claim 11 wherein said relative temperaturesare maintained by maintaining the cementation mixture at a temperaturebelow where said chromium halide will be substantially vaporized whileheating said metal body and said regeneration mass to reach saiddiffusion temperature.

"14. A method as claimed in claim 11 wherein said relative temperaturesare maintained while reaching said diffusion temperature by' maintainingthe coldest point of said metal body and regeneration mass at least 100C. above the temperature of the portion of said chamber occupied by saidcementation mixture.

15. A method as claimed in claim 14 wherein the hottest point of saidmetal body and said regeneration mass is below 500 C. above thetemperature of the portion of said chamber occupied by said cementationmixture.

16. A method as claimed in claim 14 wherein said cementation mixtureincludes no chromium and contains a halogen compound of another metalcapable of forming said other metal halide vapors which will react withsaid chromium regeneration mass to form chromium halide vapors.

17. A method as claimedin claim 11 additionally comprising circulating areducing gas which is a good heat conductor around said chamber andallowing penetration of said gas to a limited degree into said chamberthrough a leak passage in the .wall of said chamber;

18. A method as claimed in claim 17 wherein said gas includes a highpercentage of hydrogen. i

19. A method as claimedlin claim 18 additionally comprisingpositioning'lof said cementation mixture at the portion most remote fromsaid leakage passage.

20. A method as claimed in .claim 11 wherein-the relative temperaturesof said vapors and surfaces are maintained by controlling the heatingofsaid cementation mixture so as to maintain the temperature of saidhalide vapors formed below the point where there will be substantialcondensation of said vapors on said metal body and regeneration mass.

21. A method as claimed in claim 20 additionally comprising maintainingsuch controlled relationship of the temperature of said fluoride vaporsand of said metal body and said regeneration mass during the periodofcooling down of said chamber.

22. Ajmethod as claimed in claim 20 wherein the composition of saidcementation mixture is so regulated as to insure that the amount ofhalide vapors formed therefrom shall not substantially exceed theproportion required to complete the diifusion reaction.

23. A method as claimed in claim20 additionally comprising, at the endof said diffusion, cooling said cementation mixture at a rate inrelation to the temperature of said metal body and regeneration mass soas to maintain a temperature relationship between said vapors and saidmetal body and said regeneration mass such that no substantialcondensation of said vapors on said metal body and said regenerationmass will occur.

24. A method as claimed in claim 20 additionally comprising, at: the endof said diffusion, introducing a cooling body within saidcharnber nearerto said cementation mixture than said metal body and said regenerationmass, said cooling body having asubstantially lower temperature than theatmosphere of said chamber.

25. A method as claimed in claim 20 additionally comprising positioninga metal reserve between said cementation mixture and said metal body andsaid regeneration mass, and, at the end of said diffusion, introducing acooling body into said metal reserve, said cooling body having asubstantially lower temperature than the atmosphere of said chamber,said metal reserve serving to condense available vapors when diffusiondoes not occur.

26. A method as claimed in claim 25 wherein said metal reserve has athickness such that the portion thereof most remote from said coolingbody is at a temperature just sufiiciently high to prevent condensationof said vapors thereon.

27. A method as claimed in claim 20 wherein said cementation mixture isheated in the presence of sufficient water to maintain the vapor tensionof said halide below the point where any substantial condensationthereof will occur on said metal body and said regeneration mass Whilereaching said diffusion temperature.

28. A method as claimed in claim 27 wherein said water is incorporatedin an aqueous solution of a halogen acid.

29. A method of forming a superficial alloy of chromium on a metal bodyhaving as a principal component a metal selected from the groupconsisting of iron, nickel and cobalt comprising positioning said metalbody in a chamber, positioning in said chamber spaced from said body acementation mixture capable of forming chromium fluoride vapors onheating, heating said chamber to form said chromium fluoride vapors andto reach and maintain a temperature at which chromium from said vaporswill diffuse into the surface of said metal body, bringing said vaporsin contact with said metal body surface, and preventing effectivemovement ofsaid vapors into that portion of said chamber occupied bysaid metal body during the heating of said metal body to reach saiddiffusiontemperature.

30. A method as claimed in claim 29 additionally co pr g th pos t on ngQ a r ene a io mass 9 chromium in (:ofitact with, and in that portion ofsaid 2,274,671 chamber occupied by said metal. 2,604,395 2,612,442References Cited in the file of this patent UNITED STATES PATENTS 51,902,503 Howe Mar. 21, 1933 160,812 2,257,668 Becker et a1. Sept. 30,1941 14 Daeves Mar. 3, 1942 Gonser et a1 July 22, 1952 Goetzel Sept. 30,1952 FOREIGN PATENTS Australia Aug. 30, 1951

1. A METHOD OF FORMING A SUPERFICIAL ALLOY OF CHROMIUM ON A METAL BODYHAVING AS A PRINCIPAL COMPONENT A METAL SELECTED FROM THE GROUPCONSISTING OF IRON, NICKEL AND COBALT COMPRISING HEATING A CEMENTATIONMIXTURE CAPABLE OF FORMING CHROMIUM FLUORIDE VAPORS ON HEATING, HEATINGSAID METAL BODY TO REACH AND MAINTAIN A TEMPERATURE AT WHICH CHROMIUMFROM SAID VAPORS WILL DIFFUSE INTO THE SURFACE OF SAID METAL BODY,BRINGING SAID VAPORS IN CONTACT WITH, WHILE MAINTAINING SAID CEMENTATIONMIXTURE OUT OF CONTACT WITH, SAID METAL BODY SURFACE, AND MAINTAININGTHE RELATIVE TEMPERATURES OF SAID VAPORS AND SAID METAL BODY SURFACEDURING THE HEATING SAID METAL BODY TO REACH SAID DIFFUSION TEMPERATURESO AS TO PREVENT CONDENSATION OF CHROMIUM FLUORIDE ON SAID METAL BODY.